Regarding the IP phones and analog phones in the U1900 system, which of the following statements is correct ()
A . If you want to successfully register IP phones A and B to the U1900 system, the U1900 system must include the MTU board
B . When IP phones A and B have successfully registered to the same U1900 system, if IP phones A and B are able to dial each other, the U1900 system must include an MTU board
C . When IP phone A and B have successfully registered to the same U1900 system, if IP phone A calls When calling IP phone B, IP phone B is busy, then if you need IP phone A to hear busy Line tone, the U1900 must contain MTU board
D . If IP phone A and analog phone C (directly connected to the ASI board of U1900) are successfully registered to U1900, when A calls C, U1900 may not contain MTU board
Answer: C